feat(briefing): add Message.msg_metadata and RssItem.topics/classified_at columns

msg_metadata maps to the 'metadata' DB column ('metadata' is reserved
by SQLAlchemy Declarative API). to_dict() exposes the key as 'metadata'
for frontend compatibility.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/models/conversation.py

@@ -67,6 +67,9 @@ class Message(Base, CreatedAtMixin):
         Integer, ForeignKey("notes.id", ondelete="SET NULL"), nullable=True
     )
     tool_calls: Mapped[list | None] = mapped_column(JSONB, nullable=True)
+    # 'metadata' is reserved by SQLAlchemy Declarative — use msg_metadata as the
+    # Python attribute name, mapped to the 'metadata' DB column.
+    msg_metadata: Mapped[dict | None] = mapped_column("metadata", JSONB, nullable=True)
 
     conversation: Mapped["Conversation"] = relationship(back_populates="messages")
 
@@ -83,5 +86,6 @@ class Message(Base, CreatedAtMixin):
             "status": self.status,
             "context_note_id": self.context_note_id,
             "tool_calls": self.tool_calls,
+            "metadata": self.msg_metadata,
             "created_at": self.created_at.isoformat(),
         }